The invention relates to electrostatographic method and apparatus or electrostatic transfer of toner particles from a toner-image-donor roller to a receiver sheet in a transfer station, and more particularly to a time-varying transfer station current while the receiver sheet is in the transfer station.
U.S. Pat. No. 6,184,911 includes exemplary disclosure of a modular printer in which a respective secondary transfer station, included in a respective module of a plurality of tandem imaging modules, has a current regulated power supply for providing transfer station current in the respective secondary transfer station.
The Rodenberg et al. patent (U.S. Pat. No. 5,040,029) discloses a paper receiver member inserted between a photoconductive (PC) web and a transfer drum included in a multicolor electrostatographic printer, the paper receiver to be picked up by the drum. The transfer electric field is turned off for the leading edge of the receiver to aid separation from the PC web. After the last image is transferred, the transfer field is applied to the lead edge to help attach the paper receiver member to the web.
FIG. 1A illustrates an exemplary configuration of rollers in a transfer station, designated by the numeral 100. The transfer station is for electrostatic transfer and includes a toner-image-donor roller 110 including a deformable or compliant blanket 111 around a rigid core 112. Roller 110 can be for example an intermediate transfer type of roller, a photoconductor type of roller, or an electrographic imaging type of roller. Outer layers around blanket 111 which characterize the particular type of roller 110 are not shown, e.g., for usage as an intermediate transfer roller. A receiver sheet 130 is shown being transported on a transport web 135 towards a nip 140 formed between a transfer member 120 (roller or other suitable transfer member such as an electrified ski or brush for example) and roller 110, in which nip a toner image 115 carried by toner-image-donor roller is to be transferred to receiver sheet 130. Another receiver sheet 131 including a transferred image 116 is the previous in a series of N receiver sheets moved through nip 140, with receiver sheet 130 being identified as sheet (N+1). Transfer member includes a rigid core 122 and preferably a compliant layer 121 around the core. Electrostatic transfer is accomplished by providing an electric field between rollers 110 and 120 so as to urge toner particles to move from roller 110 to 120 within nip 140.
FIG. 1B illustrates the exemplary problem of unwanted wrap. Sometimes, a receiver sheet will detach from a transport web and stick to a toner-image-donor roller as the receiver sheet comes out of a transfer nip, causing a paper jam. A wrap is exemplified in the configuration 150 showing a toner-image-donor roller 155, a transfer member 165, a transport web 170 and a receiver sheet 160 partially wrapped around roller 155. The receiver sheet 160 is electrostatically adhered or tacked down to web 170. Receiver sheet 160, prior to being tacked down to web 170, tended to curl upwards, i.e., away from the web. After being adhered as depicted in FIG. 1A, such curl is largely flattened by the electrostatic tack force, yet a propensity to curl still exists near the leading edge 161 of receiver sheet 160, where the tack force can be opposed by a relatively strong curl stress
When even a small air gap forms between a receiver sheet and a transport web at the leading edge of the sheet as the sheet emerges from the transfer nip, the high electric field in the post-nip region can cause ionization of the air in this air gap. Due to the electric field, charge of one polarity will be deposited on the receiver sheet and the charge of the other polarity will be deposited on the transport web. The same electric field will cause the charge deposited on the receiver sheet to be attracted to the toner-image-donor roller, thereby attracting the receiver sheet to the intermediate roller.
In addition to curl, several other factors can contribute to producing an air gap between a receiver sheet and a transport web at the lead edge of the sheet, as the sheet emerges from the transfer nip. These other factors include paper cockle, burrs on the lead edge from cutting the sheets, receiver surface roughness, and transport web surface roughness or kinks.
A way to reduce the occurrence of wraps is to make roller 155 very small, say 50 mm diameter or less. However, this is generally disadvantageous or impractical, and a transfer member typically has a diameter of at least 150 mm so as to provide sufficient space for necessary process elements. For example, a photoconductive primary imaging roller (not illustrated in FIG. 1B) is generally used in conjunction with an intermediate transfer roller 155, with bulky process elements such as for example chargers, a toning station, a writer, and cleaners situated at various locations around the photoconductor drum and the intermediate roller, which situation demands a large diameter intermediate transfer roller.
Moreover, a typical transfer station current required for transferring a toner image to a receiver sheet is about 25 microamps, for a typical nip length (e.g., perpendicular to direction of arrow b of FIG. 1B) of about 360 mm and a transport web speed of about 300 (millimeters)(secxe2x88x921). Tests have shown that reducing transfer station current to 15 microamps or less reduces the tendency of receiver sheets to wrap on an intermediate transfer roller. However, it was found that a transfer station current this low does not produce good transfer. Therefore, simply reducing the transfer station current is not an option for avoidance of wrapping.
There remains a need to overcome the problem of unwanted wraps occurring in electrostatic transfer stations, which problem is ameliorated by the invention described below.
The invention provides apparatus and method for preventing or greatly reducing the frequency of paper jams, which can occur in a transfer station for electrostatic transfer of toner particles to a receiver sheet moving through the transfer station. As described above, such paper jams can for example result from a curl of a receiver sheet. This curl can cause a receiver sheet to wrap, thereby causing a paper jam in the transfer station.
More specifically, the invention provides an electrostatographic machine inclusive of a transfer station for electrostatic transfer of a toner image from a toner image carrier, such as a toner-image-donor roller (TIDR), to a toner-image area on a receiver sheet, the transfer station including a programmable, current regulated, power supply for purposes of producing a time variation of transfer station current for transferring the toner image. In particular, by controlling the magnitude of the transfer station current in a leading edge portion area of a receiver sheet, wrapping of receiver sheets can be reduced or eliminated.
In embodiments of an electrostatographic machine according to the invention, the receiver sheet is included in a plurality of receiver sheets successively moved through the transfer station, with toner transfer taking place in a nip formed between the TIDR and a transfer member (TR). The transfer station further includes a transport web for transporting the receiver sheet through the transfer station, the transport web being included in the nip, with the receiver sheet electrostatically adhered to the front face of the transport web, the back face of the transport web being in contact with the transfer member. The receiver sheet has a leading edge included in a leading edge margin area and a trailing edge included in a trailing edge margin area. Toner is transferred to the toner-image area hut not to a margin area. The electrostatographic machine includes a programmable power supply for supplying a transfer station current in the transfer station. During a time period between a time before the leading edge enters the nip and a time after the trailing edge leaves the nip, the transfer station current is switchably altered by the programmable power supply, by signals from a logic and control unit, between at least two predetermined magnitudes of transfer station current included in a plurality of predetermined magnitudes of transfer station current, such that at least one of the plurality of predetermined magnitudes of transfer station current causes transfer of a toner image carried on the TIDR from the TIDR to the toner-image area on the receiver sheet.
In one embodiment, the programmable power supply provides a low magnitude transfer station current, preferably zero transfer station current, prior to the time a leading edge of a receiver sheet enters the nip. At a certain time when the lead edge of the sheet is a certain distance beyond the transfer nip and the toner image area has not fully passed through the region where transfer can take place, the programmable power supply is switched so as to provide a high magnitude transfer station current suitable for transferring toner particles to the toner-image area. This suitable transfer station current magnitude is maintained until after the trailing edge is no longer in contact with the TIDR and has moved a distance past the nip, whereupon the transfer station current is switched to the low magnitude in readiness for a next receiver sheet to approach the nip.
In a preferred embodiment, the programmable power supply provides a low magnitude transfer station current, preferably zero transfer station current, prior to the time a leading edge of a receiver sheet enters the nip. At a certain time when a deformed compliant TIDR is in nip contact with the leading edge margin area but not in contact with the toner-image area, the programmable power supply is switched so as to provide a first burst of high magnitude transfer station current for a first short time interval, after which first short time interval the programmable power supply switches this high magnitude transfer station current to a suitable transfer station current for transferring toner particles to the toner-image area, which suitable transfer station current is smaller in magnitude and of the same sign as the burst of high transfer station current. This suitable transfer station current is maintained until after the trailing edge is no longer in contact with the TIDR and has moved a predetermined distance past the nip, whereupon the programmable power supply is switched so as to provide a second burst of transfer station current for a second short time interval, which second burst of transfer station current has a sign opposite to the sign of the first burst of transfer station current. At the end of the second short time interval, the transfer station current is switched to the low magnitude transfer station current in readiness for a next receiver sheet to approach the nip.
In another embodiment, a controlled time-varying reduction of transfer current magnitude within interframe time intervals between successive receiver sheets allows shorter interframe times so as to improve productivity of the electrostatographic machine. In this embodiment, the programmable power supply provides a low magnitude transfer station current, preferably zero, prior to the time a leading edge of a receiver sheet enters the nip. At a certain time when a deformed compliant TIDR is in nip contact with the leading edge margin area but not in contact with the toner-image area, the programmable power supply is switched so as to provide a suitable transfer station current magnitude for transferring toner particles to the toner-image area. This suitable transfer station current magnitude is maintained until a certain time when the TIDR is no longer in contact with the toner image area and is still in contact with the trailing edge margin area, whereupon the transfer station current is switched to the low magnitude and maintained at this low magnitude until the trailing edge has left the nip. This condition of low magnitude transfer station current is continued in readiness for a next receiver sheet to approach the nip.
In another preferred embodiment, a controlled time-varying reduction of transfer current within interframe time intervals between successive receiver sheets includes a burst of transfer station current in the interframe times so as to further improve productivity of the electrostatographic machine. In this embodiment, the programmable power supply provides a transfer station current of low magnitude, preferably zero, prior to the time a leading edge of a receiver sheet enters the nip. At a certain time when a deformed compliant TIDR is in nip contact with the leading edge margin area but not in contact with the toner-image area, the programmable power supply is switched so as to provide a first burst of transfer station current for a first short time interval, which first burst has a high magnitude. At the end of the first short time interval the programmable power supply switches the transfer station current to a suitable transfer station current for transferring toner particles to the toner-image area, which suitable transfer station current is smaller in magnitude and of the same sign as the burst of transfer station current. This suitable transfer station current is maintained until a certain time when the TIDR is no longer in contact with the toner image area and is still in contact with the trailing edge margin area, whereupon the transfer station current is switched to provide a second burst of transfer station current for a second short time interval, which second burst of transfer station current has a sign opposite to the sign of the first burst of transfer station current. At the end of the second short time interval, the transfer station current is switched to the low magnitude and maintained at the low magnitude until the trailing edge has left the nip. This condition of low magnitude transfer station current is continued in readiness for a next receiver sheet to approach the nip.
In an electrostatographic modular color printer embodiment of the invention, wherein modules are tandemly arranged for depositing single-color toner images sequentially on to a receiver sheet, each module includes a transfer station wherein the transfer station current can be switched as described above so as to reduce or eliminate paper jams in the respective transfer stations. When receiver sheets are transported through the modules without imaging, such as when clearing sheets from a machine when restarting after a paper jam, the transfer station currents provided in the modules by the respective programmable power supply outputs are preferably set to zero continuously. This produces better results than can be obtained from prior art options, which prior art options include leaving the transfer station currents at normal (continuous) operating magnitudes, setting the transfer voltages to zero, or shutting off the transfer station power supply outputs.
It should be noted that, by contrast with the Rodenberg et al. patent (U.S. Pat. No. 5,040,029), the inventors of the subject patent application have surprisingly found that turning off a transfer field at the leading edge of a paper receiver sheet helps the paper remain electrostatically adhered to a transport web, whereas Rodenberg et al. found that turning off the transfer field aided pick up of the paper from the photoconductor web by a roller.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.